You wrote a dazzling cover letter, aced multiple rounds of interviews and sent thank-you notes to everyone you met with. Now all you've got to do is sit back and wait for an offer letter, right?

Wrong.

The final, deal-sealing step in any interview process is a reference check, and failing to adequately prepare your references is a blunder bestselling management author and CNBC contributor Suzy Welch calls "really embarrassing and totally avoidable."

"At any decent organization, reference checks are critical," Welch tells CNBC Make It, which is why one of the biggest mistakes a job seeker can make is failing "to even tell their references that they're references."﻿

CNBC Contributor Suzy Welch

"It's crazy, but it happens all the time," she says. "And then what? Your references get called out of the blue, and they can be like, 'Who? What? Oh yeah, let me think for a minute.'"

To avoid this kind of embarrassing encounter Welch says you should not only alert your references to the possibility of a phone call, but prepare them for what they should discuss.

"Call your references in the day or two before they're called to let them know what skills, traits and experiences the hiring company seems to be valuing in you, so that they can emphasize those in their conversation."

For example, she says, you can say something like, "Hi there, beloved reference. The job I'm interviewing for requires quantitative analysis and collaboration, so you might want to mention my success on the ACME project."

It may feel like you're overstepping your boundaries or being too self-promotional, but Welch says you'll be surprised by how much your references appreciate the heads-up. "The last thing your references have time for is sitting around thinking about the highlights of your career. By 'educating' them with a prep call, you're actually doing them a favor."

Don't underestimate the importance of glowing — and prepared — references to help you land a position.

"If you really want that job," Welch says, "your references have to close the deal for you — and you have to help them."

Suzy Welch is the co-founder of the Jack Welch Management Institute and a noted business journalist, TV commentator and public speaker. Think you need Suzy to fix your career? Email her at gettowork@cnbc.com.